1. Field of the Invention
The present invention relates to an ink tank, a head cartridge including the ink tank and an ink jet head integrated with each other, and an ink jet printing apparatus including the ink tank and the head cartridge for performing a printing operation with them. More particularly, the present invention relates to the structure of the ink tank of the type having an ink absorbing member accommodated therein for the purpose of ink retaining.
Here, the printing operation represents all type of operations each to be performed for a various kind of ink receiving medium such as a cloth, a thread, a paper, a sheet-like material or the like so as to allow ink to be adhesively secured thereto. Therefore, the present invention can be applied to a printing apparatus, i.e., a printer serving as an information outputting apparatus operatively associated with a various kind of information processing apparatus.
2. Description of the Related Art
Many foamed blocks each molded of a polyurethane resin are hitherto used as an ink absorbing member to be accommodated in an ink tank of the foregoing type. In the case that a urethane foamed block is used as an ink absorbing member, films are formed in the foamed block during each molding operation in such a manner as to wrap each of a number of voids (pores) in the foamed block with a film. Thus, since the voids are isolated from each other due to the presence of the film between adjacent voids, the foamed block can not exhibit a function of absorbing ink therein as it is. To cope with this problem, the foamed block is subjected to film removing treatment via heating, cleaning and others. However, it is very difficult to completely remove films in the foamed block with the film removing treatment as mentioned above. In most cases, a considerable amount of residue practically adheres to each void or pore on completion of the film removing treatment.
In the case that the urethane foamed block is used as the ink absorbing member, it is usually accommodated in the ink tank in the compressed state. In addition, to assure that an adequate intensity of negative pressure acts on a communicating portion between the foamed block and a connecting member for an ink outflow portion while maintaining a certain pressure gradient across the foregoing communicating portion, a part of the foamed block is usually compressed at the communicating portion. However, since film residues remaining between adjacent voids or pores are liable to overlap in the foamed block, there arise malfunctions that ink hardly flows in the ink absorbing member, and moreover, ink fails to be fed outside of the ink tank.
On the other hand, in contrast with the urethane foamed block, an ink absorbing member comprising a foamed block molded of a condensate composed of a melamine and a formaldehyde is described in an official gazette of, e.g., International Patent Laid-Open Publication NO. WO 91/02652. The ink absorbing member as described in the above official gazette is molded in the form of a skeleton having no thin film in each gap present in the circuit network of the foamed block while assuming a net-shaped structure. Thus, the ink absorbing member composed of a melamine foamed block has many advantages that any type of film removing treatment is not required, a large quantity of ink can storably be received in the melamine foamed block owing to the presence of a number of fine fibers constituting the circuit network compared with the urethane foamed block, initial ink filling treatment can easily be conducted owing to an excellent hydrophilic property of the melamine foamed block in contrast with the urethane foamed block having a water repelling property, no ink remains in the melamine foamed block having no film formed therein due to the presence of a residue on completion of ink consumption, and the ink in the melamine foamed block can completely be utilized at a high efficiency.
Basically, it is preferable that the ink absorbing member composed of a melamine foamed block which is disclosed in the above-stated gazette is practically used in the compressed state, and ink is fed to an ink outflow portion disposed at the lower part of an ink tank by the function of the gravity force of ink itself. Thus, the ink feeding direction orienting toward the ink outflow portion is firmly determined to coincide with the downward direction. For this reason, there arises a problem that an attitude to be assumed at the time of practical use of the ink tank described in the official gazette is restrictively determined. In addition, in the case that the ink absorbing member is accommodated in the ink tank in the preferably employable uncompressed state, it is difficult that the ink absorbing member is brought in close contact with the inner wall surface of the ink tank. Thus, a gap is liable to appears between the ink absorbing member and the inner wall surface of the ink tank. When the atmospheric air taken through an atmospheric air communication port or an ink ejecting port of an ink jet head stays in the gap, there arises a malfunction that as ink is ejected from the ink jet head, a bubble is involved in the ink fed to the ink jet head, causing a quality of printed image to be remarkably degraded. Especially, with respect to an ink jet recording apparatus of the type including an ink tank and an ink jet head integrated with each other to perform a printing operation by reciprocably scanning the integrated structure composed of the ink tank and the ink jet head relative to a printing medium, there readily arises a problem that the ink tank is vibratively displaced due to the reciprocable scanning of the foregoing integrated structure. In the case that the ink jet printing apparatus is adversely affected by the vibrative displacement of the ink tank or in the case that the ink tank includes a member at the position located in the vicinity of an ink outflow portion, when a part of the ink absorbing member located in the vicinity of the ink outflow portion exhibits deterioration in terms of properties as time elapses, a gap is liable to appear at the above-noted part of the ink absorbing member. At this time, it is anticipated that the adverse influence given to the ink absorbing member due to staying of air at the gap becomes more remarkable. In an extreme case, it is preestimated that the atmospheric air communicating portion and the gap located in the vicinity of the ink outflow portion are communicated with each other. Once such a malfunction as mentioned above has arose, it becomes impossible to perform a desired ink ejecting operation, and moreover, the ink present in an ink feeding path leaks from an ink ejecting port, causing the interior of the ink jet printing apparatus to be contaminated with the leaked ink.
Since feeding of ink to the ink outflow portion is achieved by utilizing the gravity force of the ink itself, when an ink jet head is driven at a high frequency highly desired in recent years, there is a possibility that the ink feeding can not follow the driving of the ink jet head at a high frequency. To improve a property of followability of the ink jet head at the driving of the latter at a high frequency, it is thinkable that a pore size is enlarged to some extent and a magnitude of resistance against flowing of the ink is reduced. In this case, however, there is a possibility that an ink retaining capability of the ink absorbing member is degraded, causing ink to leak from the atmospheric air communicating port.
According to the description of the official gazette of the prior invention, in some case, it is desirable that a certain intensity of compressing force is applied to a foamed structure for the ink absorbing member in a specific application example of the ink jet printing apparatus in order to maintain useful or suitable properties of the ink absorbing member in the uncompressed state, and moreover, adjust a gap space of the foamed structure.
It is considered that the description of the official gazette was made in consideration of the relationship between inner dimensions of the accommodating space and outer dimensions of the ink absorbing member. The inventors of the present invention conducted a variety of examinations and as a result derived from the examinations, they found that it was acceptable that the ink absorbing member was properly compressed in order to assure that ink could smoothly and reliably be fed to the ink absorbing member regardless of an attitude assumed by the ink tank while utilizing advantages of the ink absorbing member molded of a condensate composed of a melamine and a formaldehyde. In addition, the inventors found the following technical problems to be solved. Specifically, one of the problems is that the ink absorbing member should be compressed corresponding to the structure of the ink absorbing member in a certain adequate direction in order to assure that ink can smoothly be fed to the ink absorbing member, other one is that so-called warpage or breakage is liable to occur at a compressible part of the ink absorbing member having a comparative brittle fibrous structure, and another one is that once the warpage has occurred with the ink absorbing member, the compressed state of the latter can not be maintained any more, resulting in the ink absorbing member assuming an uncompressed state.
In addition, a filter is usually disposed at the ink outflow portion for removing foreign materials involved in the ink fed from the ink absorbing member, and an opening area of the ink outflow portion is determined corresponding to a quantity of ink to be fed therefrom. However, since the thermosetting melamine based condensate is brittle in structure, a part of the condensate is peeled away from the ink outflow portion when the ink absorbing member is worked, accommodated in the ink tank or put in later practical use, and the filter is clogged with fractured pieces of the condensate. In this connection, the inventors found another technical problem to be solved at this time, i.e., a problem that a desired quantity of ink to be fed could not be assured with the ink absorbing member. These technical problems mentioned above is not described in the official gazette.